The invention relates to a paper having an improved dry strength and its method of fabrication, characterized by the use of a three-component system comprising at least two polymers mainly having cationic fillers and at least one overall anionic polymer. These polymers are combined to exert a synergistic action on the dry strength of this paper.
More precisely, the invention relates to an improved method for fabricating paper and/or cardboard and the like, in which at least three (co)polymers are used to improve the dry strength characteristics during the fabrication of cellulose sheets, respectively:                at least one first agent corresponding to a (co)polymer having a cationic filler density higher than 1 meq/g and having primary amine functions,        at least one second agent corresponding to a synthetic organic (co)polymer having a cationic filler density higher than 0.1 meq/g,        and at least one third agent corresponding to a (co)polymer having an anionic filler density higher than 0.1 meq/g.        
According to the invention, the (co)polymer having primary amine functions is conventionally obtained organic polymer and well known to a person skilled in the art, such as for example—by a Hofmann degradation reaction on a base (co)polymer—or by acidic or basic hydrolysis of a base (co)polymer of polyvinylformamide, and derivatives thereof.
The three-component system can be used successfully for fabricating papers and cardboards, coated paper supports, any type of paper, cardboard and the like requiring improved dry strength.
Ongoing efforts are being made to papers and cardboards which are increasingly strong, particularly for the packaging industry.
The dry strength of the paper is, by definition, the strength of the normally dry sheet. The values of the burst strength and tensile strength conventionally give a measurement of the dry strength of the paper.
The use of water-soluble cationic polymers for improving the strength characteristics of paper is well known. Due to their nature, they can be fixed directly on the anionic cellulose and give it a cationic filler so that in combination with anionic polymers, the latter are fixed on the cellulose fibres, thereby improving the dry strength of the sheet.
The most commonly used cationic polymers are compounds of the cationic starch type, polyamide epichlorhydrin (PAE), polyamide amine epichlorhydrin (PAAE) or cationic polyacrylamides, optionally glyoxalated.
However, the methods described in the prior art using these polymers are not fully satisfactory, particularly concerning the quantities of polymers required and/or the wet strength characteristics obtained, giving rise to process difficulties such as reduction of the sheet to pulp for recycling the dry fragments. The latter drawback is observed in particular when the method described in document US 2004/118540 is implemented.
The burst strength of the cellulose sheets obtained by the use of dry strength agents also meets a number of requirements. It must in particular have no toxicological drawbacks and have good compatibility with the other agents used in the fabrication of the sheet.
It has already been proposed, particularly in patent applications JP 58-60094 (Hamano), JP 04-57992 (Mitsui), US 2004/118540 (Kimberly Clark) to combine, in two-component type systems, polymers having vinyl amine type functions with an anionic polymer, this combination being intended to propose an efficient system for the dry strength of the paper sheet.